for the VCM effect. [ 15 , 18 , 19 ] The VCM effect may take place at dislocations that act as conducting fi laments, [ 18 ] or homogenously over a somewhat larger interface region, [ 20 ] or both within the same sample. [ 21 ] To fulfi ll the requirements of a suitable nonvolatile memory, a cell should be scalable down to a few nanometers. As a further important requirement a WRITE voltage of a few volts must be suffi cient to switch a cell within less than 100 ns and a READ voltage of a few tenths of a volt should leave the resistance state unchanged for ten years. This requirement corresponds to a nonlinear voltage acceleration of the kinetics of many orders of magnitude, also known as the voltage-time dilemma. The origin of this strong voltage acceleration has been attributed to electric-fi eld-enhanced ion-hopping mobility, although possible contributions of temperature-activated ion mobility have also been mentioned. [22][23][24][25][26][27][28] Another explanation for this effect in the literature is the electric-fi eld-enhanced recombination/generation of oxygen vacancies. [ 29 , 30 ] Herein, we report experimental results of the switching kinetics of interface-type VCM cells based on epitaxial SrTiO 3 thin fi lms as a model system, which reveal a highly nonlinear voltage-time dependency. Based on a 2D axisymmetric fi nite element simulation model, which allows a quantitative discrimination of fi eld and temperature contributions, we are able to identify the temperature acceleration as the origin of this nonlinearity. Using this model we discuss the scaling properties of the VCM cell concept and prove its feasibility on the nanoscale. Electrical CharacterizationThin-fi lm devices consisting of a SrTiO 3 (STO) thin fi lm grown epitaxially on a conducting Nb-doped SrTiO 3 (STO:Nb) and a Ti top electrode were characterized by performing quasistatic current-voltage ( I-V ) studies as well as by defi ned voltagepulse measurements. The voltage was always applied to the top electrode of the devices. The quasistatic I-V sweep was also employed to execute the initial electroforming process and to achieve a defi ned OFF state prior to the pulse studies. An overview of the setup and the measurement procedure is shown in Figure 1 . Before and after a SET voltage pulse was applied to a memory cell, its resistance was measured quasistatically to determine the resistance change. The quasistatic I-V sweep Origin of the Ultra-nonlinear Switching Kinetics in Oxide-Based Resistive SwitchesExperimental pulse length-pulse voltage studies of SrTiO 3 memristive cells are reported, which reveal nonlinearities in the switching kinetics of more than nine orders of magnitude. The results are interpreted using an electrothermal 2D fi nite element model. The nonlinearity arises from a temperature increase in a few-nanometer-thick disc-shaped region at the Ti electrode and a corresponding exponential increase in oxygen-vacancy mobility. The model fully reproduces the experimental data and it provides essential design rules for opti...

The control and rational design of redox-based memristive devices, which are highly attractive candidates for next-generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky-type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so-called deep Reset.

Resistive switching memories based on the valence change mechanism have attracted great attention due to their potential use in future nanoelectronics. The working principle relies on ion migration in an oxide matrix and subsequent nanoscale redox processes leading to a resistance change. While switching from a low resistive to a high resistive state, different intermediate resistance levels can be programmed by changing the maximum applied voltage, making resistive switches highly interesting for multibit data storage and neuromorphic applications. To date, this phenomenon, which is known as gradual reset, has been reported in various experimental studies, but a comprehensive physical understanding of this key phenomenon is missing. Here, a combined experimental and numerical modeling approach is presented to address these issues. Time‐resolved pulse measurements are performed to study the reset kinetics in TaOx‐based nano‐crossbar structures. The results are analyzed using a 2D dynamic model of nonisothermal drift–diffusion transport in the mixed electronic–ionic conducting oxide including the effect of contact potential barriers. The model accurately describes the experimental data and provides physical insights into the processes determining the gradual reset. The gradual nature can be attributed to the temperature‐accelerated oxygen‐vacancy motion being governed by drift and diffusion processes approaching an equilibrium situation.

The continuing revolutionary success of mobile computing and smart devices calls for the development of novel, cost- and energy-efficient memories. Resistive switching is attractive because of, inter alia, increased switching speed and device density. On electrical stimulus, complex nanoscale redox processes are suspected to induce a resistance change in memristive devices. Quantitative information about these processes, which has been experimentally inaccessible so far, is essential for further advances. Here we use in operando spectromicroscopy to verify that redox reactions drive the resistance change. A remarkable agreement between experimental quantification of the redox state and device simulation reveals that changes in donor concentration by a factor of 2–3 at electrode-oxide interfaces cause a modulation of the effective Schottky barrier and lead to >2 orders of magnitude change in device resistance. These findings allow realistic device simulations, opening a route to less empirical and more predictive design of future memory cells.

A new type of dielectric THz waveguide based on recent approaches in the field of integrated optics is presented with theoretical and experimental results. Although the guiding mechanism of the low-index discontinuity (LID) THz waveguide is total internal reflection, the THz wave is predominantly confined in the virtually lossless low-index air gap within a high-index dielectric waveguide due to the continuity of electric flux density at the dielectric interface. Attenuation, dispersion and single-mode confinement properties of two LID structures are discussed and compared with other THz waveguide solutions. The new approach provides an outstanding combination of high mode confinement and low transmission losses currently not realizable with any other metal-based or photonic crystal approach. These exceptional properties might enable the breakthrough of novel integrated THz systems or endoscopy applications with sub-wavelength resolution.